The separation of rare earth elements from solutions thereof wherein the elements are recovered together or from one another has become of more importance in recent years. Procedures for such recoveries have been the subject of various patents e.g. U.S. Pat. Nos. 4,647,438 and 5,015,447, which patents are incorporated herein by reference.
The procedures set forth in these patents are directed to the use of phosphine-containing extraction agents such as phosphoric acid, phosphonic acid, phosphinic acid, thiophosphinic acid and esters thereof. Additionally, phosphates such as tributyl phosphate and oxides such as trioctylphosphine oxide have been disclosed. These extractants are generally employed dissolved in a suitable organic diluent and have been shown to be especially effective when extracting rare earth elements from acidic solutions thereof. Rare earth elements are recognized as encompassing the lanthanide elements having an atomic number ranging from 57 to 71, inclusive, and yttrium whose atomic number is 39. Element 61, promethium, is, however, generally of less interest because of its radioactivity.
The general procedure employed for the separation of rare earth elements from solutions thereof, especially acidic solutions, is as follows, the feed solution generally resulting from treatment of a rare earth element containing ores such as monazite, bastnaesite, xenotime, bauxite and similar crude ores.
The feed stream of the rare earth in solution is treated in an extraction zone, and may be contacted counter-currently or co-currently, with the diluent solution of the extractant to form a raffinate and an extract. Ofttimes a neutralizing agent such as ammonia is also introduced into the feed stream to control pH. The raffinate is removed and the extract containing the extracted rare earth element(s) is usually sent to a scrubber wherein it is scrubbed with dilute acid and then sent to a stripper where it is stripped with more concentrated acid to separate the rare earth element(s). Hydrochloric acid is the preferred acid which the prior art procedures use to scrub and strip the extraction zone extract and mono-2-ethylhexyl phosphonic acid, mono-2-ethylhexyl ester is the most widely used acid extractant. Bis-(2,4,4-tri-methylpentyl) phosphinic acid is also used.
Although this method has found significantly wide acceptance in the recovery of rare earth elements, the procedure suffers from various deficiencies which, if overcome, would make the process even more commercially attractive.
One problem associated with the above-known process is that because the selectivity of the extractant employed is not as definitive as one would like, many stages of mixer-settlers are needed to achieve the desired separation. For example, some rare earth plants require 300 stages. This is in contrast to the four to eight stages required for copper recovery or the ten to twenty stages for cobalt/nickel separation. Additionally, the phosphonic acid extractant referred to above requires the use of concentrated acid for striping, e.g. 6N HCl. This results in high costs for neutralization of the excess acid and contamination of the final product by chloride ions. The chloride contamination has been overcome by the use of some prior art procedures. However, recovery of the final rare earth element(s) with HNO.sub.3 is less productive and less desirable because of the properties of nitric acid.
Therefore, a procedure whereby the above-described problems and deficiencies could be overcome or substantially reduced would solve a long-felt need.